For some years, investigations have been made for measuring the visibility on or near runways of an airport. The visibility depends on the transparency or transmissive power of the atmosphere hereinafter referred to, for greater convenience, as PTA.
The subject of the present invention is an apparatus enabling this PTA to be measured in a very precise and very simple manner.
Numerous devices are already known which are based on the response of a photosensitive cell receiving, through a certain path within the atmospheric medium to be studied, the light beam emitted by a source situated at the other end of the path, the said response naturally varying as a function of the transmissive power of the medium. The major difficulty in operating these installations, arises from the fact neither the intensity or luminance of the luminous source nor the sensitivity of the photosensitive cell are constant with time. Various automatic compensating arrangements have already been proposed for overcoming correlative errors. In particular, in French Patent Application No. 71.09954 of Mar. 22, 1971, now French Patent No. 2,130,819, the applicants proposed using an auxiliary light emitter irradiating the same photosensitive cell over a trajectory the absorption of which is independent of the fluctuations in the medium to be studied, the luminosities of the main and auxiliary sources being mutually controlled in such a manner that the fluxes received by the cell originating from the two emissions, are at a constant ratio. For correcting variations in the sensitivity of the cell, such a device also comprises a control arrangement between the said cell and the auxiliary source so as to maintain the light emission from the auxiliary source in inverse proportion to the sensitivity of the cell.
Technically, the above system gives every satisfaction. However, for proper operation, it requires the presence of a qualified maintenance staff. The frequent absence at small airports of specialised staff for maintaining such equipment necessarily means that the maintenance operations much be less frequent and as simple as possible at site level.